"The short-term financial benefits of reopening are far outweighed by the long-term benefits of fully protecting our customers," said Henrik Hasselknippe, a managing director at the Green Exchange – which is backed by Goldman Sachs and other big investment banks and which has refused to restart trading in its "daily futures" contract.

"We still do not feel we have an overview of the situation. The Austrian government has so far refused to publish a list of missing allowances and we will continue to be worried until they do so," he added.

The ongoing scandal is an embarrassment for the EU, which regards the scheme as a key tool in the climate change strategy it is trying to encourage other countries to adopt. The "cap and trade" system, launched six years ago, aims to limit greenhouse gas output from 11,000 power and industrial plants.

The level of unease about varying standards across the EU was highlighted last week when Barker wrote to Connie Herdegaard, the EU climate commissioner, urging her to ensure "urgent and significant" action is taken.

The Belgian court case has been brought by TCEI of Italy, a holding company for trading house, The Cube Energy, which is hoping to recover 267,991 allowances that were stolen.

Laurent Arnauts of Arnauts Solicitors in Belgium which is acting for TCEI, told the pointcarbon.com website that it hoped the judge would freeze the stolen allowances. "We know the credits are stolen, but not where they are. Our main goal is to recover them." The EU is confident it will win the case.

Austria – one of a number of markets hit by the cyber attacks – says it has tracked down the stolen allowances but has so far declined to make public the serial numbers or confirm it has had them returned. It has cited national data protection laws, while governments across Europe are trying to work out whether companies found to hold stolen allowances – even unwittingly – could be subject to legal action.